A variety of semiconductor laser driving circuits are known previously by including one illustrated in FIG. 5 for controlling the intensity of light emitted from a laser diode used in an image-forming apparatus. (Japanese Patent No. 3332916, for example.)
Referring to FIG. 5, a semiconductor laser driving circuit 100 is configured to drive a laser diode LD such that the intensity of light emitted therefrom reaches a predetermined value.
The semiconductor laser driving circuit 100 includes at least a photodiode PD, an amplifier circuit 102, a sample hold circuit 103, a current generator 104, an on-off circuit 105, a detection circuit 106, and resistors R101 and R103.
The amplifier circuit 102, sample hold circuit 103, current generator 104, on-off circuit 105, and detection circuit 106 are all integrated into one single integrated circuit (IC).
In the amplifier circuit 102, an external signal Sext is inputted to the non-inverting terminal and an output signal from the detection circuit 106 is inputted to the inverting terminal. In addition, an output signal from the amplifier circuit 102 is outputted to the sample hold circuit 103.
The anode of the photodiode PD is connected to the input terminal of the detection circuit 106 and a photocurrent generated by the photodiode PD is inputted to the input terminal.
The output terminal of the detection circuit 106 is connected to the non-inverting terminal of the amplifier circuit 102, and to the R101 which operates to convert the photocurrent into a voltage.
In addition, the sample hold circuit 103 consists of an analog switch AS and a capacitor HC, and a sample hold signal Ssh is inputted to the control electrode of the analog switch AS.
The sample hold signal Ssh is configured to turn on the analog switch AS to be electrically continuous, only when the on-off circuit 105 is switched on by an image signal Sp.
When the analog switch AS is turned on, the capacitor HC is charged by the output voltage from the amplifier circuit 102.
In contrast, when the analog switch AS is turned off, the capacitor HC is charged by the output voltage from the amplifier circuit 102 to be held as a charged voltage of the capacitor HC.
The current generator 104 is configured to generate a current to be proportional to a sample-held voltage, or the voltage caused by the charge in the capacitor HC. This current serves as a driving current for the laser diode LD.
The constant for the proportionality of current-to-voltage conversion is determined by the resistor 103 which is connected between the terminal 123 and the ground.
The switching circuit 6 is configured to be on-off controlled according to the inputted image signal Sp so as to intermittently supply the current outputted from the current generator 104 to the laser diode.
Therefore, in the semiconductor laser driving circuit 100 previously known, the capacitor HC for the sample hold circuit 103 is externally provided and the response time for light intensity control in the feedback control loop is adjusted by suitably selecting the capacitance value for the capacitor HC Japanese Patent No. 3332916).
Because of the capacitor HC externally connected to IC, a difficulty encountered in the previous laser driving circuit is that the point of grounding IC cannot be placed in close vicinity of that of the capacitor HC, and that, as a result, a slight difference in ground potential arises due to the current flowing through grounded circuits from one grounding point to another, which may be notable particularly in the laser driving circuit dealing with pulse currents.
In addition, considerable noises tend to be generated both inside and outside IC. The IC terminals, to which external devices are connected, may be affected by various interferences with relative ease through parasitic capacitances derived between groundings, power sources, and signal conductors.
As a result, driving currents for the laser diode unduly fluctuate and the control of light intensity cannot be carried out with satisfactory accuracy in the previous laser driving circuit.
It is therefore desirable to provide a semiconductor laser driving circuit having improved capability of controlling laser light intensity with satisfactory accuracy.